Exhaust gas purifying filter

ABSTRACT

There is disclosed an exhaust gas purifying filter of the present invention comprising a honeycomb structure formed of a porous ceramic material and including: a large number of through-holes  3 ; plugged portions  10  formed in one end portion of each of predetermined through-holes  3  and in the other end portion of each of remaining through-holes  3;  and partition walls  2  partitioning the through-holes  3  and formed into filter layers, characterized in that a plugging length is non-uniform for each plugged portion  10 , and the filter has characteristics such as high porosity and thin wall while hardly causing defects such as chipping and falling-off of the plugged portions during packing, transporting, and handling.

TECHNICAL FIELD

The present invention relates to an exhaust gas purifying filter for usein removing solid particulates in an exhaust gas, which is representedby a diesel particulate filter.

BACKGROUND ART

To remove solid particulates containing carbon as a main component froma combustion exhaust gas exhausted, for example, from a diesel engine, afilter for purifying an exhaust gas (hereinafter referred to simply asthe “filter”) has heretofore been used in which one-end portions ofpredetermined through-holes of a honeycomb structure formed of a porousceramic material and including a large number of through-holes areplugged, the other end portions of the remaining through-holes areplugged, and partition walls partitioning the through-holes are formedas filter layers.

FIG. 6 is a schematic diagram showing a plugged state of a filter endsurface. Plugged portions 10 constituted by charging a plugging materialinto end portions on opposite sides in adjacent through-holes (cells) 3(shown in FIG. 5) are alternately formed to close the through-holes(cells) 3 in such a manner that the end surface of the filter has acheckered pattern in the shown state. FIG. 5 is a schematic diagramshowing a plugging structure of a conventional filter for purifying theexhaust gas. The plugged portions 10 are formed in the one-end portionsof the through-holes (cells) 3 and the other end portions of theremaining through-holes (cells) 3, and accordingly partition walls 2partitioning the through-holes (cells) 3 bear functions of filterlayers.

In recent years, resulting from technical progress of the diesel engine,especially high pressure injection of fuel, miniaturization of solidparticles exhausted from the diesel engine has advanced, and there hasbeen an increasing expectation for development of a diesel particulatefilter (DPF) for use in capturing such fine solid particulates.

Moreover, there has been a growing interest in an environmental problem,restrictions on the exhaust gas have also became tighter, and a filterwhich is capable of more effectively capturing the solid particulatesand which has characteristics such as high porosity and thin wall hasbeen developed at fever pitch.

Although the increasing of the porosity of the partition wall and thethinning of the wall are realized, there is a tendency that strength ofthe partition wall decreases, and therefore a problem easily occurs thatdefects such as chipping and cracking are easily caused also by a slightimpact load. Therefore, careful attentions need to be paid during notonly packing and transporting of products but also canning process andhandling of the products.

Especially, boundaries between the plugged portions 10 into which theplugging material is charged and the through-holes (cells) 3 into whichthe plugging material is not charged in FIG. 5 are sometimes cracked,and accordingly the defects such as chipping and falling-off of theplugged portions are generated in many cases. This is supposed to be aphenomenon resulting from a difference in strength between the pluggedportions 10 and the through-holes (cells) 3, but this problem has becometangible with the increasing of the porosity of the partition wall 2 andthe thinning of the wall, and there has been a demand for a concretecountermeasure.

The present invention has been developed in consideration of theproblems of the related arts, and an object thereof is to provide anexhaust gas purifying filter which has characteristics such as highporosity and thin wall but which hardly causes defects such as chippingand falling-off of plugged portions during packing, transporting, orhandling.

DISCLOSURE OF THE INVENTION

That is, according to the present invention, there is provided anexhaust gas purifying filter comprising a honeycomb structure formed ofa porous ceramic material and including: a large number ofthrough-holes; plugged portions formed in one end portion of each ofpredetermined through-holes and in the other end portion of each ofremaining through-holes; and partition walls partitioning thethrough-holes and formed into filter layers, characterized in that aplugging length is non-uniform for each plugged portion.

On the other hand, according to the present invention, there is providedan exhaust gas purifying filter comprising a honeycomb structure formedof a porous ceramic material and including: a large number ofthrough-holes; plugged portions formed in one end portion of each ofpredetermined through-holes and in the other end portion of each ofremaining through-holes; and partition walls partitioning thethrough-holes and formed into filter layers, characterized in that aplugging length in a central portion of the plugged portion is formed togradually shorten as compared with that in an outer peripheral portionof the plugged portion.

In the present invention, the plugging length of the through-hole in acentral part of the exhaust gas purifying filter is preferably formed togradually shorten as compared with that of the through-hole in an outerperipheral part of the exhaust gas purifying filter.

The above-described honeycomb structure of the present invention ispreferably formed of at least one material selected from the groupconsisting of cordierite, zirconium phosphate, aluminum titanate,lithium aluminum silicate (LAS), and silicon carbide.

Moreover, in the present invention, a catalyst is preferably supported,and the catalyst preferably contains at least one type selected from thegroup consisting of Pt, Pd, Rh, K, Ba, Li, and Na.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing one embodiment of a filter forpurifying an exhaust gas according to the present invention;

FIG. 2 is a schematic diagram showing another embodiment of the filterfor purifying the exhaust gas according to the present invention;

FIG. 3 is a schematic diagram showing one embodiment of a structure of aplugged portion of the filter for purifying the exhaust gas according tothe present invention;

FIG. 4 is a schematic diagram showing another embodiment of thestructure of the plugged portion of the filter for purifying the exhaustgas according to the present invention;

FIG. 5 is a schematic diagram showing a plugged structure of aconventional filter for purifying the exhaust gas;

FIG. 6 is a schematic diagram showing a plugged state of a filter endsurface; and

FIG. 7 is a schematic diagram showing a method of performing an impactstrength test.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described hereinafter, butit should be understood that the present invention is not limited to thefollowing embodiments and that modifications, improvements and the likeof designs are appropriately added without departing from the scope ofthe present invention based on ordinary knowledge of a person skilled inthe art.

In a filter for purifying an exhaust gas in the present invention, oneend portion of each of predetermined through-holes of a honeycombstructure formed of a porous ceramic material and including a largenumber of through-holes is plugged, the other end portion of each ofremaining through-holes is plugged, and partition walls partitioning thethrough-holes are formed as filter layers. In the plugging, in the samemanner as in a conventional known diesel particulate filter, as shown inFIG. 6, a plugging material is charged into end portions on oppositesides in adjacent through-holes (cells) 3 (shown in FIG. 5) toalternately form plugged portions 10 in such a manner that an endsurface of the filter has a checkered pattern, and the through-holes(cells) 3 are preferably closed.

When the exhaust gas is passed from one end surface of the filter, theexhaust gas containing solid particulates flows into the filter fromthrough-holes whose end portions on this end surface side are notplugged, passes through porous partition walls, and enters the otherthrough-holes whose end portions on the other end surface side of thefilter are not plugged. Moreover, the solid particulates in the exhaustgas are captured by the partition walls during the passing through thepartition walls, and the purified exhaust gas from which the solidparticulates have been removed is exhausted from the other end surfaceof the filter.

FIG. 1 is a schematic diagram showing one embodiment of the filter forpurifying the exhaust gas according to the present invention, and showsa state in which the plugged portions 10 are formed in one end portionof the through-hole (cell) 3 and the other end portion of the remainingthrough-hole (cell) 3. In this manner, the filter of the presentembodiment is characterized in that a plugging length of each pluggedportion 10 becomes non-uniform. That is, a boundary between the pluggedportion 10 in which the plugging material is charged and thethrough-hole (cell) 3 in which the plugging material is not charged isnon-uniform for each through-hole (cell). Therefore, the boundary is noteasily cracked, and accordingly an effect is produced that defects suchas chipping and falling-off of the plugged portion are not easilygenerated.

Next, another embodiment of the filter according to the presentinvention will be described. The filter of the present invention ischaracterized in that a plugging length in a central portion of theplugged portion is formed to gradually shorten as compared with that inan outer peripheral portion of the plugged portion.

FIG. 3 is a schematic diagram showing one embodiment of a structure ofthe plugged portion of the filter for purifying the exhaust gasaccording to the present invention, and shows a state in which aplugging length in a central portion X of the plugged portion is formedto gradually shorten as compared with that in an outer peripheralportion Y of the same plugged portion in one end portion of thethrough-hole (cell) 3.

It is to be noted that the “outer peripheral portion” of the pluggedportion mentioned here means a portion abutting on the partition wallconstituting the through-hole and its vicinity in a case where the endsurface of the filter is observed in parallel with the through-hole. The“central portion” of the plugged portion means a portion in the vicinityof a middle of the through-hole and inside the above-described “outerperipheral portion”.

In this manner, the filter of the present embodiment is constituted insuch a manner that strength of the filter gradually changes in theboundary between the plugged portion 10 in which the plugging materialis charged and the through-hole (cell) 3 in which the plugging materialis not charged. Therefore, the boundary is not easily cracked, andaccordingly an effect is produced that the defects such as the chippingand falling-off of the plugged portion are not easily generated.

FIG. 4 is a schematic diagram showing another embodiment of thestructure of the plugged portion of the filter for purifying the exhaustgas according to the present invention. In one end portion of thethrough-hole (cell) 3, the plugging length in the central portion X ofthe plugged portion is formed to gradually shorten as compared with thatin the outer peripheral portion Y of the same plugged portion, and aportion in which the plugging material is not charged is formed into apyramid shape in this example. Even when the shape of the pluggedportion is changed in this manner, needless to say, an equivalent effectis produced that the defects such as the chipping and falling-off of theplugged portion are not easily generated.

FIG. 2 is a schematic diagram showing another embodiment of the filterfor purifying the exhaust gas according to the present invention, andshows a state in which the plugged portions 10 are formed in one-endportions of the through-holes (cells) 3 and the other end portions ofthe remaining through-holes (cells) 3 in the same manner as in FIG. 1.However, in the present embodiment, the plugging lengths of thethrough-holes 3 in a central part A of the honeycomb structure ispreferably formed to gradually shorten as compared with those of thethrough-holes 3 in an outer peripheral part B of the honeycombstructure. That is, even in this embodiment, the boundary between theplugged portion 10 in which the plugging material is charged and thethrough-hole (cell) 3 in which the plugging material is not chargedbecomes non-uniform for each through-hole (cell), therefore the boundaryis not easily cracked, and accordingly the effect is produced that thedefects such as the chipping and falling-off of the plugged portion arenot easily generated.

It is to be noted that the “outer peripheral part” of the honeycombstructure mentioned in the present invention means the vicinity of eachouter peripheral wall 20 of the honeycomb structure (exhaust gaspurifying filter 1) in a case where the end surface of the honeycombstructure (exhaust gas purifying filter 1) is observed in parallel withthe through-hole shown in FIG. 6. The “central part” of the honeycombstructure is a part inside the above-described “outer peripheral part”.

When the exhaust gas of a diesel engine is passed from one end surfaceof the filter, the exhaust gas flows into the filter from holes whoseend portions on this end surface side are not plugged, passes throughporous partition walls, and enters the other holes whose end portions onthe other end surface side of the structure are not plugged. Moreover,the solid particulates in the exhaust gas are captured by the partitionwalls during the passing through the partition walls, and the purifiedexhaust gas from which the solid particulates have been removed isexhausted from the other end surface of the filter.

It is to be noted that when the captured solid particulates areaccumulated on the partition walls, the filter is clogged, its functionis deteriorated, the filter is periodically heated by heating means suchas a heater to burn/remove the solid particulates, and the filterfunction is regenerated. However, to promote the burning of the solidparticulates at this regeneration time, the filter may support acatalyst. The catalyst is preferably a catalyst containing at least onetype selected from the group consisting of Pt, Pd, Rh, K, Ba, Li, and Nafrom a viewpoint of efficient regeneration of the filter function.

Moreover, in the filter including the honeycomb structure whose cellsare plugged in a checked pattern as in the present invention, both acell opened on an inflow side and a cell opened on an exhaust side mayalso be coated with ceramic particle layers. However, since the solidparticulates in the exhaust gas are accumulated on the former cell, onlythe former cell is preferably coated from a viewpoint of minimization ofa pressure loss rise by the coating.

A material constituting the honeycomb structure forming a main body ofthe filter of the present invention is preferably at least one materialselected from the group consisting of cordierite, zirconium phosphate,aluminum titanate, LAS, and silicon carbide. Cordierite, zirconiumphosphate, aluminum titanate, and LAS have small coefficients of thermalexpansion. Therefore, when one material selected from the groupconsisting of them, or a compound material mixed with two or more ofthese materials is used as the material constituting the honeycombstructure, a filter superior in thermal shock resistance is obtained.When zirconium phosphate, aluminum titanate, and silicon carbide areused as the materials constituting the honeycomb structure, a filtersuperior in heat resistance is obtained because their melting points arehigh. When the plugging material for use in plugging the through-hole(cell) of the honeycomb structure matches the material constituting thehoneycomb structure, both coefficients of thermal expansion preferablymatch each other.

It is to be noted that the filter according to the present inventionhardly causes the defects such as the chipping and falling-off of theplugged portion during packing, transporting, and handling as comparedwith the filter including a conventional structure of the pluggedportion. Therefore, an effect is also produced that cost required formaintenance after manufacturing the filter is reduced.

Next, details of the filter of the present invention will be describedin accordance with an example of a method of manufacturing the filter.

A basic structure of the filter including the honeycomb structure isformed by an extrusion molding method. The basic structure mentionedherein refers to a cell structure, that is, rib thickness, cell density,diameter, and length, and they are formed at the time of the extrusionmolding; In general, in consideration of a ratio of a dimension at themolding time to that after calcining, the molding may be performed insuch a manner that a desired dimension can be achieved after thecalcining.

After heating/drying a honeycomb structure formed article obtained inthis manner on appropriate conditions, plugging is performed in order toimpart the filter function. It is to be noted that there are a casewhere the plugging is performed after the forming/heating and anothercase where the plugging is performed after the calcining, but eithercase may be used.

Next, an example of a general method of charging the plugging materialwill be described. A thin film opened in a checkered pattern is attachedto the end surface of the honeycomb structure, the structure is immersedinto a slurry plugging material from the end surface, and slurry isallowed to enter the through-holes of honeycomb corresponding to filmopenings to charge the plugging material. The plugging lengths may becontrolled by a depth of the honeycomb structure immersed into theslurry.

To charge the plugging material in uniform lengths with respect to allthe through-holes, as described above, the film may be attached and thestructure may be immersed into the slurry once. However, to obtain thefilter including the structure represented by FIGS. 1, 2 according tothe present invention, the immersing operation may be performed twice ormore. That is, to perform the immersing twice or more, for example, athin film including two or more opening patterns of the central part andouter peripheral part of the end surface of the honeycomb structure isprepared, and a filter including a honeycomb structure having adistribution of the plugging lengths can be obtained, when this film isused, and the immersing depth is changed.

As another method, the central portion of an area of the film opening isreduced, and the peripheral portion thereof is enlarged to control anamount of slurry which flows into the through-hole. Even by this method,the filter including the honeycomb structure having the distribution ofplugging lengths can be obtained.

Moreover, to obtain the plugged portion structures shown in FIGS. 3, 4,a rubber film opened in a checkered pattern is attached to the endsurface of the honeycomb structure, and the slurry plugging material maybe poured from the end surface. In this case, when an opening area ofthe rubber film is changed, the amount of slurry flowing into each cellcan be adjusted, and accordingly the plugged portion structure can beformed as shown in FIGS. 3, 4.

The present invention will be described hereinafter in more detail basedon examples, but the present invention is not limited to these examples.

EXAMPLES 1 TO 10

Clay was prepared by a conventional known method, and a honeycombstructure was formed having a diameter of 145 mm, length of 152 mm, wallthickness of 0.31 mm, and cell density of 300 cells/square inch by anextrusion molding method. Next, after drying an obtained molded article,a plurality of thin films having different opening patterns were used,end portions of through-holes were sealed with a plugging materialidentical to the material of a main body in order to achieve a plugginglength distribution shown in Table 1, and this article was calcined at1400° C. to prepare a filter including a honeycomb structure formed ofcordierite. It is to be noted that the “plugging length (mm) in Table 1means a plugging length in each part, assuming that a middle part is(A), a position of 24 mm from the middle part is (B), a position of 48mm from the middle part is (C), and an outermost peripheral part is (D),when the end surface of the filter is observed in parallel with thethrough-hole. It is to be noted that in Table 1, porosity (%) andaverage pore diameter (μm) of the filter indicate values measured by amercury porosimetry.

EXAMPLES 11 TO 13

Filters including honeycomb structures formed of zirconium phosphate,aluminum titanate, and LAS were prepared in procedures similar to thoseof Examples 1 to 10 described above.

EXAMPLES 14, 15

Filters including honeycomb structures formed of glass combined SiC wereprepared in procedures similar to those of Examples 1 to 10 describedabove.

It is to be noted that a sintered body of an SiC material can beobtained by calcining, but a calcining temperature is generally high at2000° C. or more. On the other hand, when a glass forming material isadded to a pure SiC raw material and calcined, a glass softeningtemperature is low, the calcining at the low temperature is possible,and a micro structure of the obtained sintered body is a two-phaseporous structure in which raw material SiC is partially combined in aglass phase. In the present example, 15% by mass of a cordierite rawmaterial was added to SiC which was a raw material and used, and thefilters including the honeycomb structures formed of glass combined SiCwere prepared.

EXAMPLES 16, 17

Filters including honeycomb structures formed of cordierite wereprepared by procedures similar to those of Examples 1 to 10 describedabove except that a plurality of rubber films including openings havingsmall opening areas as compared with thin films and including differentopening patterns were used instead of the plurality of thin films.

COMPARATIVE EXAMPLES 1 TO 5

Filters including honeycomb structures formed of cordierite wereprepared by methods similar to those of Examples 1 to 10 except that theplugging length was set to be equal with respect to all through-holes.

COMPARATIVE EXAMPLES 6 TO 8

Filters including honeycomb structures formed of zirconium phosphate,aluminum titanate, and LAS were prepared by methods similar to those ofExamples 11 to 13 except that the plugging length was set to be equalwith respect to all through-holes.

COMPARATIVE EXAMPLES 9, 10

Filters including honeycomb structures formed of glass combined SiC wereprepared by methods similar to those of Examples 14, 15 except that theplugging length was set to be equal with respect to all through-holes.

(Impact Strength Test)

With respect to the respective filters of Examples 1 to 17 andComparative Examples 1 to 10, as shown in FIG. 7, an impact was added toan end part 16 of a filter 1 in a side surface direction by use of ahammer 15 of a Charpy tester. The impacted filter was visually observedto evaluate a degree of wear and tear, that is, damage. Results areshown in Table 1. It is to be noted that as evaluation standards, “⊚” ina case where any damage was not generated, “◯” in a case where thedamage was little, “Δ” in a case where the damage was of a mediumdegree, and “x” in a case where the damage was remarkable were describedin “evaluation of damage with respect to impact” in Table 1.

TABLE 1 Evaluation Average Plugging length of damage Porosity porediameter (mm) with respect Material (%) (μm) (A) (B) (C) (D) to impactExample 1 Cordierite 60 20 5 5 7.5 10 ◯ Example 2 Cordierite 60 20 5 510 20 ⊚ Example 3 Cordierite 60 20 5 5 7.5 20 ⊚ Example 4 Cordierite 6725 5 5 7.5 10 ◯ Example 5 Cordierite 67 25 5 5 10 20 ⊚ Example 6Cordierite 67 25 5 5 7.5 20 ⊚~◯ Example 7 Cordierite 60 20 10 10 20 40 ⊚Example 8 Cordierite 67 25 10 10 20 40 ⊚ Example 9 Cordierite 63 22 5 57.5 20 ⊚ Example 10 Cordierite 63 22 10 10 20 40 ⊚ Example 11 Zirconium50 20 5 5 7.5 10 ◯ phosphate Example 12 Aluminum 45 20 5 5 10 20 ⊚titanate Example 13 LAS 45 20 5 5 7.5 20 ⊚ Example 14 Glass 40 10 5 57.5 10 ⊚ combined SiC Example 15 Glass 45 15 5 5 10 20 ⊚ combined SiCExample 16*¹ Cordierite 67 25 5 5 7.5 10 ⊚~◯ Example 17*² Cordierite 6725 5 5 7.5 10 ⊚~◯ Comparative Cordierite 54 15 5 5 5 5 ◯~Δ Example 1Comparative Cordierite 60 20 5 5 5 5 Δ~X Example 2 ComparativeCordierite 67 25 5 5 5 5 X Example 3 Comparative Cordierite 60 20 10 1010 10 Δ~X Example 4 Comparative Cordierite 67 25 10 10 10 10 Δ~X Example5 Comparative Zirconium 50 20 5 5 5 5 Δ~X Example 6 phosphateComparative Aluminum 45 20 5 5 5 5 Δ Example 7 titanate Comparative LAS45 20 5 5 5 5 Δ~X Example 8 Comparative Glass 50 10 5 5 5 5 Δ~X Example9 combined SiC Comparative Glass 45 15 5 5 5 5 Δ Example 10 combined SiC*¹A plugged portion has a shape shown in FIG. 3, and a ratio (x/y) of aplugging length (x) in a central portion X to a plugging length (y) inan outer peripheral portion Y is 0.5. *²A plugged portion has a shapeshown in FIG. 4, and a ratio (x/y) of a plugging length (x) in a centralportion X to a plugging length (y) in an outer peripheral portion Y is0.7.

As apparent from the results shown in Table 1, the filters of theexamples according to the present invention had less damages on the endparts, that is, the plugged portions with respect to the impacts ascompared with the filters of the comparative examples, and it waspossible to confirm superiority of the present invention.

Moreover, even when the filters were prepared using zirconium phosphate,aluminum titanate, LAS, and glass combined SiC as the materials otherthan cordierite, it was possible to confirm that the filters hadsuperior impact resistances.

INDUSTRIAL APPLICABILITY

As described above, according to an exhaust gas purifying filter of thepresent invention, since a plugged portion is formed into apredetermined structure, the filter has characteristics such as highporosity and thin wall, but produces an effect of hardly causing defectssuch as chipping and falling-off of the plugged portion during packing,transporting, and handling as compared with an exhaust gas purifyingfilter including a conventional plugged portion structure. Reduction ofcost required for maintenance of the filter during the packing,transporting, and handling is also possible.

1. An exhaust gas purifying filter, comprising: a honeycomb structureformed of a porous ceramic material having a first and second axial endsand defining a plurality of through-holes, a plurality of through-holesbeing sealed in one end portion with a plug to form a predeterminedpattern of sealed through-holes at the first axial end, and theremaining through-holes being sealed in the second axial end portionwith a plug; and partition walls partitioning the through-holes andformed into filter layers; wherein: the length of each plug isnon-uniform; and the length of the plugs gradually shorten in astep-wise manner from an outer peripheral portion of an axial end of thefilter to a central portion of the axial end.
 2. An exhaust gaspurifying filter, comprising: a honeycomb structure formed of a porousceramic material having a first and second axial ends and defining aplurality of through-holes, a plurality of through-holes being sealed inone end portion with a plug to form a predetermined pattern of sealedthrough-holes at the first axial end, and the remaining through-holesbeing sealed in the second axial end portion with a plug; and partitionwalls partitioning the through-holes and formed into filter layers;wherein each plug is configured so that one end of the plug tapers inlength inwardly so that a length of the plug at an outer diameter islonger than a length of the plug at an inner diameter; and the length ofthe plugs gradually shorten in a step-wise manner from an outerperipheral portion of an axial end of the filter to a central portion ofthe axial end.
 3. The exhaust gas purifying filter according to claim 1,wherein the honeycomb structure is formed of at least one materialselected from the group consisting of cordierite, zirconium phosphate,aluminum titanate, LAS, and silicon carbide.
 4. The exhaust gaspurifying filter according to claim 2, wherein the honeycomb structureis formed of at least one material selected from the group consisting ofcordierite, zirconium phosphate, aluminum titanate, LAS, and siliconcarbide.
 5. The exhaust gas purifying filter according to claim 1,further comprising a catalyst.
 6. The exhaust gas purifying filteraccording to claim 2, further comprising a catalyst.
 7. The exhaust gaspurifying filter according to claim 5, wherein the catalyst contains atleast one element selected from the group consisting of Pt, Pd, Rh, K,Ba, Li, and Na.
 8. The exhaust gas purifying filter according to claim6, wherein the catalyst contains at least one element selected from thegroup consisting of Pt, Pd, Rh, K, Ba, Li, and Na.